<!DOCTYPE html>
<html>

	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>

	<body>
		<script type="text/javascript">
			function change() {
				//document.getElementById("sel").selected = true;
				document.getElementById("sel").value = 'qwe';

			}
		</script>
		<select id="sel">
			<option value="1">1</option>
			<option value="2">2</option>
			<option value="3">3</option>
			<option value="qwe">qwe</option>
		</select>
		<input type="button" value="修改" onclick="change()" />
	</body>

</html>